In both the OnePage and Conference configurations you have the option to upload video from previous events to show your attendees before your event is live.
In your event, enable editing mode and then scroll down to find the gallery section.
If you haven't already you need to create categories for your videos to be placed into.
Click on the edit icon and then create and organise your categories.
Once you have created the categories you are happy with, click on the Add Video icon in the content area, and then fill in the information.
Here you can add the title, content, a custom thumbnail, the speakers on the session (you can also update their profile as well), and you can set what you want to show in the sidebar. If you do not want anything you can live that area blank.